The monitoring identified some regular issues for the nationally endangered penguins, such as avian diphtheria in chicks and predation injuries at sea, DoC's Coastal Otago operations manager Annie Wallace says.

"Like last season, this year there have been some unexplained adult deaths," she said.

"Further investigation into these deaths indicate signs and symptoms which are consistent with previous events. In 2013, 67 hoiho were found dead and in 2017 six were confirmed to have died, all in similar circumstances."

Despite research into the reasons for these deaths - which includes sending samples to Wildbase at Massey University - and the ruling out of many possibilities, the exact cause has not yet been identified, she said.